How does the DP4000 series squelch system work, and why is it so good?

How does the DP4000 series squelch system work, and why is it so good?

Description of the squelch system differences between traditional analog radios and Motorola DP4000 series radios

Squelch on traditional analog radios
On most analog radios the squelch system is quite an unsophisticated “noise gate” that will only allow audio to go from the demodulator to the speaker of the radio if the incoming received signal is above a certain level of strength. There is usually “hysteresis” built in so that, if the received signal level drops slightly the radio will continue to output that signal to the audio section.

Let us use the example of a radio that is designed to unmute (direct the audio to the speaker) at -115dBm (0.399µv), and mute again at -118dBm (0.282µv). There is a hysteresis of 3dBm or 0.117µv. In this example, a signal of stronger than .399µv will cause the radio to unmute. If the signal varies anywhere between .282µv and higher, the radio will continue being unmuted. Only when the signal drops below .282µv will the radio speaker be muted.

The main reason for this relatively high level needed to unmute the radio is to prevent false triggering of the squelch system which causes the radio to sporadically unmute when the radio passes some form of low-level interference such as electric generators, arc welders, power pylons etc.

Because this system is unsophisticated and there is no way for the radio to determine that a signal weaker than .282µv is maybe actually a valid signal and not just noise, the radio rejects all signals below that level.

Squelch on Motorola DP4000 series radios

The audio system used on Motorola DP4000 series radios is designed very differently from the above description. In these radios the demodulated audio does not go directly to the speaker via the squelch circuit, but it first passes through a sophisticated Digital Signal Processing circuit that is designed to recognise voice patterns even in high noise environments. It is able to remove almost all unwanted noise, and will only unmute the radio speaker if it determines that the signal is valid, though extremely weak.

The result of this is that the Motorola DP4000 series radios can unmute at a signal as low as around-123dBm (0.159µv) and still manage to faithfully resolve valid audio information down to around -127dBm (.1µv).

Compared to a traditional analog squelch system, the Motorola is able to be around 12dB more sensitive AND far better able to reject interference and false triggers. All signals that fall between .399µv and .159µv will not be heard on traditional radios, but will be heard on the DP4000 series. This extra sensitivity is a significant improvement in the performance of the radio as can be seen in the below diagram.

Not only will weaker signals be resolved by the Motorola, but greater communications distances can be achieved as well. A 12dB improvement in sensitivity can mean several extra kilometres of coverage.